School Science Lessons
Plant cells and tissues
2015-07-04 sp
Please send comments to: J.Elfick@uq.edu.au
Table of contents9.51.0 Plant cells and tissues9.52.0 Leaves9.53.0 Roots9.54.0 Stems9.70.1 Plants in dry environments9.51.0 Cells and tissues9.9.0 Cells, plant cells, Elodea9.0.1 Plant tissue types9.54.2 Phloem9.0.2 Plant tissues, plant parts9.54.1 XylemExperiments9.9.2 Plasmolysis, Elodea, Tradescantia9.57 Cells and tissue sections, T.S., L.S., V.S., R.L.S., T.L.S.9.56.1 Effect of temperature and chemicals on beetroot plasma membrane9.55 Human cheek cells9.61 Microscope staining techniques9.9.3 Onion leaf scale cells, onion leaf epidermis, bulb9.58 Parenchyma cells of tomato9.59 Phloem cells of pumpkin, Cucurbita9.54 Plant cell, cork cells, Robert Hooke9.9.4 Plant epidermis, Tradescantia, Zebrina9.9.5 Subsurface sections of leaf, Vinca9.9.7 Stone cells, pear9.60 Section cutting by hand9.9.1 Stamen hair cells, Tradescantia9.64 Wood cells, Eucalyptus, poplar9.52.0 Leaves, (Experiments)9.53.6 Cladode, Bossiaea, (GIF)1.6 Different leaves, (Primary)9.5.1a External features of a leaf, elm, beech, apple, Hydrangea9.5.6 Foliage leaves, stipules9.68 Isobilateral leaf, Eucalyptus9.67 Grass leaf3.27 Leaf classification, (Primary)3.26 Leaf collection, (Primary)9.82 Leaf of Hakea, xeromorphic leaf5.07 Leaf, leaves9.5.4 Leaf, stomate, apple, adaptations of stomates9.5.5 Leaf tendrils, garden pea, sweet pea6.6.8 Leafy crops9.70 Leaves containing aerenchyma, water lily9.70.1 Plants in dry environments, Acacia, Opuntia5.31 Leaves lose water, (Primary)9.66 Leaves of agricultural plants9.65 Leaves of bushy plants9.5.11 Phylloclades, butcher's broom9.5.12 Phyllode, Acacia9.69 Stomates in a leaf9.66.3 Structure of dicotyledon leaf, privet, lilac9.69.1 Structure of stomates, Eucalyptus, Hakea9.53.0 Roots6.6.4 Starchy root crops6.6.7 Tap root crops and bulb cropsExperiments
9.3.17 Adventitious roots, twig of the willow9.73.1 Apogeotropic roots, mangroves9.3.18 Climbing adventitious roots, English ivy9.71.0 Dicotyledon root and monocotyledon root9.3.7 Dicotyledon root, broad bean, buttercup9.74 Excretion of acids by roots9.73 Gravity affects the growth of stems and roots9.3.6 Lateral roots, cress, coconut9.72 Legume roots, broad bean, clover, Rhizobium9.3.9 Mycorrhizal roots, birch, pine, heather, bird's nest orchid5.10 Root hairs9.3.3 Root hairs, cress9.75 Root hairs of germinating common bean plant6.9.16.7 Root nodules4.3.13 Root nodules, Isolate micro-organisms, Rhizobium4.3.12 Root nodules, Nitrogen-fixing bacteria9.3.15 Root pressure, Fuchsia, busy Lizzie9.184 Root pressure, plant Water transport, root pressure5.10.01 Root rhizosphere9.76 Root structure of mung bean5.30 Roots absorb water, (Primary)9.77 Roots absorb water, Tradescantia1.29 Roots and stems, (Primary)5.09 Roots, bushy plants, grasses9.3.5 Roots, cress, mustard9.97 Roots from plant parts9.3.19 Specialized roots, prop roots, tap roots, tuberous roots9.3.12 Storage roots with food reserves, potato9.3.16 Tap roots, wallflower, groundsel9.66.2 Young root, black mustard, white mustard9.54.0 Stems5.08 Woody stems, wood9.78 Celery stalk, bean stem9.185 Conduction of water and salts through stems 9.6.22 Corm, false stem (pseudostem), banana, taro9.79 Dicotyledon stem, sunflower9.6.22 Corm, false stem (pseudostem), banana, taro9.1.2.1 Corm, gladiolus, crocus9.6.14 Creeping stems, moneywort (creeping jenny), ground ivy 9.6.20 Herbaceous dicotyledon stem, buttercup9.6.6 Herbaceous dicotyledon stem, carnation 9.6.7 Herbaceous monocotyledon stem, iris9.6.5 Herbaceous stem, forage legume alfalfa, (lucerne)9.80 Monocotyledon stem, maize, (corn), Zea mays9.80.1 Monocotyledon stems, Cocos nucifera coconut, Dracaena9.1.4.1 Rhizome, ginger, iris 9.6.15 Runners, strawberry3.28 Stems and roots, (Primary) 9.6.18 Stem hooks, bramble (blackberry), rose 9.6.16 Stolons, currant, European gooseberry, banana 9.6.13 Terminal bud, linden tree (lime tree), beech, oak 9.6.8 Xeromorphic stem, spinifex 9.6.12 Twigs of trees in winter, horse chestnut, sycamore, linden tree (lime tree), beech, oak 9.6.21 Twining stem, climbing bean, yam 9.6.17 Woody stem, hawthorn9.54.1 Xylem9.57 Cells and tissue sections, T.S., L.S., V.S., R.L.S., T.L.S.9.0.1 Plant tissue types (See: 8. Xylem)9.3.7 Dicotyledon root, broad bean, buttercup9.54.2 Phloem9.57 Cells and tissue sections, T.S., L.S., V.S., R.L.S., T.L.S.9.0.1 Plant tissue types (See: 9. Phloem)9.3.7 Dicotyledon root, broad bean, buttercup9.59 Phloem cells of pumpkin, Cucurbita9.63 Stem with secondary thickening, linden tree (lime tree), horse chestnut9.0.2 Plant tissues, plant partsSee diagram 9.53: Plant parts
The tissues are formed by groups of cells that have similar or related
functions.In multicellular plant bodies the cells are cemented together
where adjacent cell walls touch by the middle lamella that may contain
calcium pectate.Where the walls are not in contact, the spaces between
them form a continuous intercellular system of air spaces.This can be very
extensive in parenchyma tissues, or absent in conducting tissues.The plasmodesmata,
fine cytoplasmic connections through minute pores in walls of adjacent cells,
maintain continuity of the cytoplasm.When the walls become thickened, these
connections are often lost.Pits, or thin areas in cell walls, are sometimes
associated with plasmodesmata.In many differentiated cells, the cell walls
become thick and impregnated with other substances and the protoplast disappears
at
maturity.These non-living units are still called cells.They function
in conduction of materials (xylem) provide mechanical strength (fibres)
or give protective covering (cork and bark).Tissue arrangement is different
in animals where the cells have no walls and are embedded in a matrix secreted
by the cells.This matrix is often extensive and lacks an air space system
such as is found in plant tissues.A plant tissue may be simple or complex.
A simple tissue is a group of structurally similar cells of similar origin
and performing the
same function, parenchyma (storage and packing) collenchyma
(mechanical) sclerenchyma (mechanical).A complex tissue is a group of
dissimilar cells of similar origin performing the same function, vascular
tissue of the xylem containing:
vessels, fibres, tracheids, and parenchyma
cells.9.3.3 Root hairs, cress
| See diagram 9.75: Root hairs on germinating bean
seed
| See diagram 9.73.3: Root hairs in the
soil
| See diagram 9.73.2: Root hairs in L.S. root
1. Root hairs increase the surface area of the roots and usually occur
in very large numbers, Maize, Zea mays has 420 root hairs
per mm2.Leaf cress,
garden cress seeds, Lepidium sativum, to swell in water in a flat
glass dish for 15 minutes.Cut a square of filter paper from a sheet and
wrap around a glass plate and hold in place with rubber bands.Transfer
swollen cress seeds with forceps to the filter paper and put in two rows,
each row 3 cm from the edge of the two narrow
ends.Because their coats
are mucilaginous, the seeds stick well to the paper.The glass plate is
placed in a 400 mL beaker, water is added until the level almost reaches
the lower row of seeds.The beaker is then covered with a Petri dish and
left to stand.Within three days small cress seedlings have developed from
the seeds.The roots of the bottom row that are dipping in the water have
almost no root hairs.The roots of the upper row growing in air on the moist
filter paper have formed a large number of hairs.The roots on the lower
row have formed few roots.9.3.5 Roots, cress, mustard
1. Soak a ceramic flower pot in water and scatter the cress seeds thinly
over the inner surface.They stick to the pot because of their mucilaginous
seed coats.Invert the pot over a dish containing enough water to cover
the rim of the pot and put in a warm place.The seeds will quickly germinate
and provide suitable roots for examination.Cut off the terminal 1 cm from
the end of one root and mount in lactophenol-erythrosin.Note the arrangement
of tissues at the apex, the region of elongation, the development and structure
of root hairs.After making these observations, crush the specimen and examine
the older region for annular and spiral vessels of the protoxylem.
2. Germinate mustard seedlings on damp absorbent paper and cover with
a glass jar.
Cut transverse sections:
1. At 1 mm behind tip for apical meristematic cells and root cap cells.
2. At 2 to 4 mm behind tip for elongating cells and 3. 6 to 8 mm behind
tip for elongated cells and root hairs.Stain in acid phloroglucin and mount
in 50% glycerine solution.(Mustard and cress seedlings is a favourite salad
sandwich filling.)9.3.6 Lateral roots, cress, coconutSee diagram 53.5: Lateral roots of coconut
Allow cress seedlings or other seedlings to grow until the radicle shows
lateral roots.Cut off the radicle just above the smallest visible laterals
and mount this terminal length in lactophenol-erythrosin, coiling it round
if
necessary.Note the stages in the development of the lateral roots.
Coconuts have no root hairs.9.3.7 Dicotyledon root, broad
bean, buttercup
| See diagram 9.73.1: Root types
| See diagram 9.73.2: Root L.S.| See diagram 9.73.3: Root in soil
| See diagram 9.71.2: Root transverse sections,
TS
1. Cut a transverse section of a broad bean root.Use the thumb and forefinger
to hold the root between two pieces of pith or carrot tissue.Dip the material
in water to moisten it.Never cut dry material! To cut microscope sections
use a one-sided razor blade, "Gem", dipped in water.Hold the material vertically
and draw the razor blade quickly across it.Cut the thinnest possible sections
as wedge shapes.Wash the sections into a small dish of water.Use a camel
hair paint brush to select the thinnest section, mount it in
water on a slide and cover with a coverslip.Irrigate with aniline sulfate
to colour the xylem elements yellow.Look for the following tissues all
embedded in parenchyma.Note the relative positions of the various tissues.
Examine the tissues under high power and note the cellular structures:
1.1 the piliferous layer, with its root hairs.
1.2 the cortex, composed of the cortex proper and the endodermis.
1.3 the stele, composed of xylem (protoxylem and metaxylem) phloem and
pericycle.
2. Cut a transverse section of a buttercup root.Note the creeping stems
and the adventitious roots arising at nodes from stems.Observe the epidermis,
wide cortex containing parenchyma, and the endodermis that is the innermost
layer of the cortex.The endodermis forms a definite ring of thickened cells.
See the passage cells through the endodermis opposite the protoxylem.The
phloem and xylem are exarch, i.e. the first cells to differentiate are towards
the outside of the stele.The xylem consists of four protoxylem groups linking
with the metaxylem that occupies the centre.In the root, the first cells
of the xylem to become fully differentiated, i.e. lignified, are those
cells towards the outside of the core of
xylem.This part of the xylem,
differentiated while the root is still elongating, is the protoxylem.Metaxylem
is the remainder of the xylem to differentiate after the root stops elongating.
Protoxylem and metaxylem together form the
primary xylem.Primary xylem and
primary phloem are both derived directly from the provascular tissue.9.3.9 Mycorrhizal roots, birch,
pine, heather, bird's nest orchid
Mycorrhizal roots are found growing in the surface layers of leaf mould
below the trees.Note the characteristic branching.Cut sections of leaf
mould to see ectotrophic mycorrhiza.Collect young, thin roots of heather
in the spring.Mount a length of one of them in lactophenol-erythrosin and
look for endotrophic fungal threads in the narrow cortex.Examine a section
of heather root to see endotrophic mycorrhiza.9.3.12 Storage roots with food
reserves, potatoSee diagram 9.85: Potato tuber
Test seeds, leaves, roots, stems, tubers for glucose and fructose.9.3.15 Root pressure, Fuchsia,
busy LizzieSee diagram 9.184: Root pressure
Do NOT use elemental mercury for school experiments!
1. Cut a Fuchsia stem.Cut the stem of a single stem pot plant
1 cm above the soil.Fit a short piece of rubber tubing to the cut stump
then fill the tubing with water.Insert into the end of the rubber tubing
50 cm of narrow bore glass tubing.Fix the glass tubing vertically and each
day measure the height of liquid in it.Replace the glass tubing with a manometer
and record the root pressure attained.
2. The salt concentration of the cell sap in both the root hairs and
root cells is generally higher than that of their surroundings.Consequently,
because of the semipermeable nature of the cell membranes, water is absorbed
by osmosis through the root and rises
in the plant under a certain pressure.
This root pressure maintains the turgor of plant tissues.Cut a busy Lizzie
or a Fuchsia, horizontally, 5 cm above the soil.Apply glycerine
around the outside of the stump.Fix a piece of rubber tubing over it and
bind with string.Connect a glass tube, 400 mm long, to the other end of the
rubber tubing and hold it in place with a small clamp and a right angle
clamp
attached to a Bunsen burner stand.Attach a scale to the glass tube.Pour
water into the tube so that its level can be read against the scale.Covert
the tube with liquid paraffin to prevent evaporation from the surface of
the water.Add water to the pot regularly and read the level of the water
meniscus in the glass tube.The meniscus slowly rises in the glass tube.9.3.16 Tap roots, wallflower,
groundselSee diagram 9.73.1: Two kinds of roots
Any grass has fibrous roots.Watercress has adventitious roots.In dicotyledons,
the main axis of the root (tap root) grows vertically downwards and is continuous
with the stem.It bears lateral branch roots that originate from within the
root tissue.The tip of each root has a growing apex protected by a root
cap.Several centimetres behind the apex is a mass of root hairs growing
out from the surface layer of the root.These increase the surface area of
the roots where water absorption occurs.In monocotyledons, grasses.
the fibrous root system with no obvious tap root.There are many roots
of about equal size and most of them arise from the lower part of the stem
at nodes (adventitious roots).The shoot system consists of a stem with
long leaves attached at nodes.The leaves have no petioles, but have long
sheathing leaf bases that in the young plant enfold the younger leaves.At
the top of the sheathing leaf base there may be a small membranous tongue,
the ligule, a common feature of grasses.The lamina is long and linear with
parallel venation.9.3.17 Adventitious roots, twig
of the willow
Observe the development of adventitious roots on the twig of the willow.
Gather the twigs late in the winter and place in water.In a few weeks,
the buds will open and adventitious roots will appear on the stem.Keep a
dated record of the progress of the shoot.9.3.18 Climbing adventitious
roots, English ivy
Detach a spray of ivy from an old wall or the trunk of a tree.Note
how difficult doing this without breaking the stems of the ivy is because
the adventitious roots cling to their support.Examine these adventitious
climbing roots.9.3.19 Specialized roots, prop
roots, tap roots, tuberous rootsSee diagram 9.87: Sweet potato tuber
1. Prop roots holds up the stem, maize (corn)
2. Swollen tap roots store food, radish
3. Tuberous roots store food, sweet potato9.5.1a External features of
a leaf, elm, beech, apple, HydrangeaSee diagram 9.66: Papaya leaf
Choose a simple form of leaf and examine its external appearance in detail.
Note the leaf, showing the swollen leaf base, the petiole
and the lamina.
Examine the type of venation, and note how the veins gradually diminish
in size, until the ultimate branches are scarcely
visible.9.5.4 Leaf, stomates, apple, adaptations
of stomates
Iris and Narcissus have an isobilateral monocotyledon leaf with palisade
tissue on both surfaces.The thickness of the cuticle varies on different
leaves of the same plant.However, plants adapted to dry conditions, e.g.
Hakea and Eucalyptus, have thick cuticles and plants growing
with abundant water
supply, Nymphaea, water lily, have thin cuticles.9.5.5 Leaf tendrils, garden pea,
sweet pea, (Yellow vetchling has stem tendrils.)
Note their position and relation to other leaf structures.9.5.6 Foliage leaves, stipulesSee diagram 9.66.2: Different leaves | See diagram 52.2: Breadfruit stipules | See diagram 53.3: Coconut leaf
Note the stipules of certain leaves in rose, pea.Note net venation
(reticulate venation) and parallel venation in grasses.Leaves adapted
for photosynthesis have the following features:
1. The broad leaf blade, lamina, has a large surface area to volume
ratio and many stomates, pores, to help light absorption and gas
exchange.
2. Many vein endings supply water and remove sugar from the leaf.9.5.11 Phylloclades, butcher's
broom
Note the very reduced leaves that are modified branches.9.5.12 Phyllode, AcaciaSee diagram 9.53.5:Acacia
Note the buds or branches in the axils of the phyllodes showing that
these are modified leaf structures.
9.54 Plant cell, cork cells,
Robert HookeSee diagram 9.54: Plant cell
Robert Hooke, (1635-1733), examined very thin slices of cork.He noted
compartments that reminded him of cells, the small rooms used by monks
in monasteries.He was looking at the dead cell walls.The cavities of the
compartment previously contained the living cells.
Experiment
1. Cut a wedge-shaped thin slice of cork and pith from the centre of
a stem, e.g. potato, watermelon, tomato.Observe the cells as seen by Robert
Hooke.
2. Cut a wedge-shaped thin slice of a soft green stem.Look for cytoplasm,
nucleus, chloroplasts, cell wall, and cell membrane.9.9.0 Cells, plant cells,
Elodea, pond weedSee diagram 9.63: Elodea cells | See diagram 9.54: Plant cell details
Plant and animal cells are similar, consisting of a protoplast bounded
by a cell membrane.However, plant cells have a rigid cellulose wall surrounding the protoplast.
The cell wall is in contact with its cell membrane.Mature plant cells
have vacuoles and plastids.Plastids are membrane bound organelles in the
cytoplasm.The three types of plastids are as follows:
1. Chloroplasts contain chlorophyll pigments and occur in all green
parts of the plant.
2. Chromoplasts contain carotene and xanthophyll pigments.They give
colour to all red, orange and yellow parts of the plant.The pink, purple
and blue colour of plants come from anthocyanin pigments dissolved in the
vacuole sap, e.g. Tradescantia, beetroot.
3. Leucoplasts are colourless plastids found in most other plant cells
where starch grains may form as a storage product, potatoSolanum tuberosum.
Experiment
1.
Mount a complete leaf of Elodea in water on a slide and examine
under high power of the microscope.Note the small green granules containing
chlorophyll, chloroplasts.Observe the movement of the chloroplasts showing
that the cytoplasm is moving, cytoplasmic streaming.Note the cellulose
cell walls.
2. Contrast plant cells with animal cells, e.g.
9.55 Human cheek cells9.9.1 Stamen hair cells of TradescantiaTradescantia, is an important plant for study because the
purple hairs on the stamens are "self staining".So it is possible to observe
the movements of the contents of a live plant cell with the contents already
stained.Use forceps to remove a stamen from a Tradescantia young
flower.Pull off one purple hair from the stamen, mount it in a drop of
water, apply a coverslip and examine it under low power.
Note the following:
1. The cuticle that sheaths the cell and whole filaments of cells.
2. The cellulose cell wall around each cell.
3. The middle lamella layer between neighbouring cells only.
4. The peripheral cytoplasm containing organelles that give it a granular
appearance.
5. The nucleus in the peripheral cytoplasm or suspended by cytoplasmic
strands in the centre of the vacuole.
6. The vacuole containing dissolved substances, e.g. purple anthocyanin
pigments.
7. The movement of granules in the cytoplasm, cytoplasmic streaming.
8. The movement of the nucleus.Check the position of the nucleus in
the same cell every five minutes.9.9.2 Plasmolysis,
Elodea, TradescantiaSee diagram 9.63: Cell, waterweed, Elodea
cellSee diagram 9.178: Plasmolysis in Tradescantia
cells
1. Use plasmolysis to show that the cell wall is a non-living envelope
distinct from the cytoplasm.Irrigate the preparation with a hypertonic
salt solution more concentrated than the cell sap.By osmosis, water passes
from the weaker solution to the stronger solution through the differentially
permeable living cell membranes.The resulting contraction of the living
cell contents is called plasmolysis.In the plasmolysed cell, the cell
wall that is permeable to the solution remains rigid.The salt solution remains
in the space between the cell wall and protoplast.If you then irrigate
the preparation with water the protoplasts expand again quite rapidly as water
passes in through the living
membranes of the cytoplasm to the vacuole.The
original state of turbidity of the cells returns.
2 To study the differentially permeable membranes, irrigate the preparation
with iodine to kill it and again attempt plasmolysis with
hypertonic salt
solution.The cell no longer responds because the differentially permeable
properties of the cytoplasmic membranes have died.A mature plant cell with
vacuoles has two cytoplasmic membranes.The outer membrane is in contact
with the cell wall.The inner membrane separates the vacuole from the cytoplasm.
Use iodine solution to stain the nucleus.
3. Examine the small leaves near the end of the stem of the waterweed
Elodea.Put a single small leaf in a drop of water on a glass microscope
slide, cover with a coverslip and examine with a microscope.In strong light
the cellular contents may have a flowing motion called cytosis or protoplasmic
streaming.
4. Make a slide of living Elodea to show the presence if a cell
wall.Put a drop of salt water solution on one edge of the coverslip.Draw
the salt solution under the coverslip by placing a piece if absorbent towelling
or blotter at the opposite side of the slip so that the
liquid on the slide
will rise up the paper.Water will diffuse out of the cells into the salt
water.As this proceeds, the cellular contents may be observed to shrink,
but the rigid cell walls retain their original structure.Other plant cells
may be used to show this phenomenon.Fleshy leaves with a thin layer that
can be peeled off are possible sources of thin cellular layers.
5. Repeat the experiment with Tradescantia, lettuce and spinach
cells.9.9.3 Onion leaf scale cells,
onion leaf epidermis, bulbSee diagram 9.56: Plant cell (diagrammatic) |
See diagram 2.30: Detach epidermis from leaf
A.cell wall, B.middle lamella, C.nucleus, D.cytoplasm, E.plasma
membrane, F.tonoplast (plasma membrane) G.vacuole containing cell sap
1. An onion bulb is a condensed shoot with a very short stem enclosed
by fleshy leaf bases, leaf scales.Cut an onion bulb in half, longitudinally
(downwards).Use forceps to peel off the thin epidermis from the concave
(inner) side of an onion leaf scale.Put a small flat piece of it in a
water drop on a microscope slide.Apply a coverslip and examine the structure
of the cells under low power.Stain the cell contents by putting one drop
of iodine solution at the edge of the coverslip, then draw the solution under
the coverslip by
putting absorbent paper on the opposite edge.Note the
cytoplasm enclosing several large vacuoles and the normally colourless nucleus
now pale yellow because of the iodine solution.Note the thin cellulose cell
wall surrounding the whole cell.
2. Methyl green acetic acid solution and carmine acetic acid solution
simultaneously fix and stain.Transfer a drop of methyl green acetic acid
to a slide with a glass rod.Detach a small piece of epidermis from the
inner side of a scale of an onion.Put it immediately into the drop of methyl
green acetic acid.Apply a coverslip and examine the preparation at a magnification
of 250 X.The cell nuclei will be stained a strong blue-green colour, while
the cell walls will only be weakly tinted.The rest of the cell contents
remain unstained.The image in the microscope will be even more contrasting
if, after the desired intensity of staining has been reached, the dye solution
is
replaced by 2% acetic acid.Apply a drop of 2% acetic acid to one edge
of the cover glass with a glass rod, and suck it under the cover glass
by applying a piece of
filter paper to the opposite side.If carmine acetic
acid is used, the cell nuclei are stained a deep red.Use methyl green
acetic for more fragile plant specimens and for showing the nuclei of protozoa.9.9.4 Plant epidermis, Tradescantia,
ZebrinaSee diagram 9.69.3: Stomates, surface view,
guard cells | See diagram 9.65.3.1: Section
view of a leafSee diagram 9.69: Stomates guard cells
1. Use a razor blade to make an incision on the under surface of a Tradescantia
leaf and strip off a small section of epidermis.Mount outer surface uppermost
in water.Tradescantia has no stomates in the upper epidermis.
Observe the following:
1.1. The guard cells that should be open if the weather is bright.
1.2. The extra thickening on the walls of the guard cells is next to
the pores.
1.3. Accessory cells surround the guard cells.
1.4. The larger epidermal cells have colourless plastids called leucoplasts.Guard cells
have chloroplasts but epidermal cells do not.Leucoplasts cluster around the nucleus of the accessory cells.
2. Plants possess a "skin" called the epidermis.To study the epidermis
of the leaf of a trailing plant, transfer a drop of water from a beaker
to a microscope slide with a glass rod.Stretch a trailing plant leaf, with
the lower surface facing upwards, over the index finger of the left hand,
holding it in place with the
middle finger.Make a small cut on the surface
of the leaf with a dissecting needle.Grasp the torn edge with pointed forceps
and peel off a small piece of the epidermis of the lower surface of the
leaf.Put it, with the external surface upwards, in the drop of water on
the slide.Mount a coverslip.Examine the slide under low power.Note the
shape of the epidermal cells and the bean shape cells that always lie together
in pairs, called "guard cells".A stomate is this pair of cells and the
opening between them.Most of the water lost by the plant, and all the exchange
of gases takes place through these stomates.The guard cells of the stomates
may be surrounded by four cells that differ from that of the other epidermal
cells.This group of six cells is called a stomatesl apparatus.Find a stomatesl
apparatus.Examine the slide under high power.Note the stomatesl apparatus
and the epidermal cells surrounding it.Note which epidermal cells contain chloroplasts.Observe the cell nuclei.9.9.5 Subsurface sections of
leaf, Vinca
Use a variegated leaf, e.g. Vinca, to cut thin subsurface sections
just below the upper and lower epidermis.To do this, hold a leaf over your
index finger and anchor it with your thumb and third finger.
Observe the following:
1. The upper epidermal cells and the tops of the palisade cells.The
air spaces between palisade cells are small in diameter but extend vertically
through the tissue.Mount this section with epidermis uppermost.
2. The lower epidermal cells and part of spongy mesophyll cells with
large air spaces.Mount this section with the spongy mesophyll uppermost.9.9.7 Stone cells, pearSee diagram 9.78.4: Stone cells
Pull apart the gritty tissue of pear fruit on a slide.Examine the structure
of the stone cells.Note the absence of cytoplasm and look for simple unbranched
and branched pits.Stain with iodine solution or aniline sulfate or aniline
chloride to show the lignified walls.9.55 Human cheek cellsSee diagram 9.3.67: Human cheek cells.Note
the nucleus, cytoplasm, plasma membrane or plasmalemma, and granules.
You may have to seek approval to do this experiment because saliva can
carry disease.Instead of taking cheek cells you can use prepared slides
of cheek cells from a school laboratory supplier.
Cheek cells come from the stratified squamous epithelium tissue on the
surface of the mucous membrane inside the cheek.These flat, scale-like
cells are shed constantly as the tissue is renewed so it is easy to obtain
some for study by gently scraping the inside
of the cheek.This tissue is
not keratinized so the surface cells are still living and have live nuclei,
in contrast with shed epidermal cells.Similar tissue lines the vagina.
Experiment1. Observe human epithelial cells from inside the cheek.Use a clean toothpick
to gently scrape the inside surface of the cheek.Put the whitish scraping
into a drop of water or 0.65% saline solution on a microscope slide.Add
a drop of stain, e.g. methylene blue or iodine solution, and apply a coverslip.
View under low power and high power.Note the protoplasm containing a central
nucleus and granular cytoplasm.The outer boundary of the protoplasm is
the plasma membrane.Adjacent cells look like paving stones.The nucleus and cytoplasm have a different refractive index, so note the
interfaces between them.In later experiments, compare the animal cell with
the plant cell.The animal cell has no cell wall.
2. Contrast animal cells with plant cells, e.g. 9.9.0 Cells, plant cells,
Elodea9.56.1 Effect of temperature and chemicals on beetroot plasma membrane
See: Spectroscopes, Spectrophotometer,
"Scientrific", (commercial website)
This experiment is an indirect study of
the effects of different substances and treatments on living beetroot cells.
Each beetroot cell has a large central vacuole bounded by a plasma membrane.
The vacuole contains the red pigment anthocyanin that
gives the beetroot
its typical colour.The beetroot cell is also surrounded by the cell membrane.
If both membranes remain intact, the anthocyanin cannot escape into the
surrounding environment.If both membranes are stressed or damaged, lysis occurs and the red
colour can leak out.The cell wall surrounding plant cells provides a structure
to the plant.It does not have a role in controlling the movement of substances
into and out of cells.The dependent variable is the colour of the beetroot cells.
The manipulated
variables could be temperature, or the concentration of
chemicals, e.g. alcohol.
If you have a spectrophotometer the max = 535 nm.
What is the maximum temperature a cell can sustain before lysis occurs? How
does alteration of temperature impact on the structures of a cells? How can the addition of alcohol or saline chemicals
to the environment impact on the cell membrane stability?9.57 Cells and tissue sections,
T.S., L.S., V.S., R.L.S., T.L.S.
| See diagram 9.57: Tissue sections 1
| See diagram 9.57.1: Tissue sections 2
| See diagram 9.57.2: Wood section
A slice across a stem, at right angles to the axis of the stem, is
a transverse section, T.S.Any slice parallel to the axis of a stem is
a longitudinal section, L.S.A slice vertically down is a vertical section,
V.S.A slice parallel to the axis of the stem, along the radius, is a
radial longitudinal section, R.L.S.A slice parallel to the axis of the
stem, along a tangent to the cross-section, is a tangential longitudinal
section, T.L.S.
Experiment1. Cut a wedge-shaped transverse section across a soft stem, e.g. tomato,
potato, sunflower.Note the groups of similar cells, tissues.The epidermis
is the one cell thick outer layer.It may have a waxy cuticle on the outside
to protect against desiccation.The bundles of cells, vascular bundles,
contain food conducting phloem cells on the outside and water conducting xylem
cells on the
inside.The walls of the xylem cells, vessels, are strengthened.
Old xylem forms wood.Groups of cells with very thick walls, sclerenchyma,
strengthen the stem.Parenchyma tissue is the loose packing cells.Between
the xylem and the phloem are closely packed cells with large nuclei and thin
walls, the cambium.Cambium cells produce new cells by mitosis to make the
stem thicker.Draw a map diagram to show the different tissues.
2. Observe the remaining stump of a cut down tree or the sawn end of
a thick branch.Note the sap wood, heart wood, annual rings, phloem and
bark.The appearance of the rays shows the type of section.In transverse
section, T.S., the rays are radial lines often only one cell in width.
In radial longitudinal section, R.L.S., the rays appear
as partial brick
walls.Any broken appearance is caused by the section not being exactly radial.In tangential longitudinal
section, T.L.S., the rays appear as lens-shaped areas and from this type
of section the actual vertical extent
and width of the rays may be accurately
determined.L.S., longitudinal section, refers to any section at right
angles to the axis.Examine the T.S., R.L.S. and T.L.S. sections of the
wood of the linden tree (Tilea europea).Find the rays and identify the type of section.9.58 Parenchyma cells of tomatoSee diagram 9.58: Parenchyma cells of tomato
1. Look for the thin cell wall, plasma membrane, vacuole, cytoplasm,
chromoplasts, and nucleus.
2. Remove a very small portion of the pulpy tissue immediately beneath
the skin of a tomato fruit.Mount this on a slide in water and then tease
it out with dissecting needles.Apply a coverslip and examine under high
power.Note the parenchyma cells containing orange-red chromoplasts and
cytoplasm, nucleus and vacuoles.Stain with iodine solution and examine
the structure in detail.9.59 Phloem cells of pumpkin,
CucurbitaSee diagram 9.59.1: T.S.Pumpkin stem | See diagram 9.59.2: T.S. Vascular bundleSee diagram 9.59.3: L.S. and T.S. phloem cells,
high power
1. Note the collenchyma, cortex, endodermis, pericycle, pith (often
broken), parenchyma (packing tissues), phloem, xylem, cambium,
bicollateral
vascular bundle (phloem both outside and inside xylem), characteristic
of pumpkins and melons.
2. Examine a transverse section and a longitudinal section.Note the
general arrangement of the bicollateral bundles, with the phloem both internal
and external to the xylem in the vascular bundles.Sieve tubes form vertical
files of cells placed end to end.Where each cross wall is perforated is
called a sieve plate.Each sieve tube element has a companion cell next to
it.Companion cells are small with dense cytoplasm.9.60 Section cutting by handSee diagram 9.60: Cut sections by hand
1. Use a razor blade, preferably one-sided, to cut a very thin slice
from a cork or a stick of pith. Be careful! Cut away from the body! Examine
the slice with a magnifying glass or low power microscope.Cut an incomplete
shaving, like a thin slice of cheese, and examine its thinnest edge.Note
the arrangement of the dead cell walls.
2. Cut a transverse section, T.S., at right angles to the long axis
of the organ or plant.Cut a longitudinal section, L.S., parallel to the
axis of the organ or plant.Cut a radial longitudinal section, R.L.S.,
as with a longitudinal section but cut along the radius of the organ or
plant.
3. Make a transverse section by cutting a carrot or piece of pith in
half longitudinally.Then hold the tissue to be sectioned between the two
halves of the carrot or pith and cut across, away from you, with a one-sided
razor blade, e.g. "Gem".9.61 Microscope staining techniquesSee diagram 2.26: Drawing stain across specimen
under coverslip 3.13.1 Haematoxylin solution, Delafield's
haematoxylin, microscopy stain
Use safety glasses and nitrile chemical-resistant gloves when working
with stains.
1. Irrigation
Mount a section of plant tissue in a drop of water on a microscope slide.
Put a coverslip on the drop of water so that no air bubbles
remain under
the coverslip.Put a drop of stain near the edge of the coverslip so that
it is in contact with the edge of the drop of water.Touch the other side
of the drop of water under the coverslip with absorbent paper to draw the
stain across the plant tissue.
2. Immersion in iodine solution
Put sections of plant tissue in iodine solution for one minute.Remove
the sections, rinse in tap water and mount them on a microscope slide in
dilute glycerine.
3. Immersion in acid phloroglucin solution
Put sections of plant stem in phloroglucin solution for 30 seconds.
Use a mounted needle to transfer the section to a drop of concentrated
hydrochloric acid for two seconds.Mount the section in glycerine and apply
a coverslip.Observe the lignin in xylem or woody tissue stained bright
red.
4. Immersion in safranin and haematoxylin solutions
Put sections of plant tissue in 50% by volume alcohol / water solution.
Use a mounted needle to transfer sections to safranin solution.Wash sections
with tap water.Transfer sections to 's haematoxylin solution.Observe the
section under a microscope to monitor the staining.If the section is overstained,
destain in acidified alcohol solution.Wash sections, mount in glycerine
and apply a coverslip.9.63 Stem with secondary
thickening, linden tree (lime tree), horse chestnutSee diagram 9.57.4: Tilia TS Stem
1. Examine transverse sections through the stem of various ages as follows:
1.1 near the apex of the shoot.
1.2. the middle of the first year's growth.
1.3. near the bottom of the first year's growth.
1.4. about the middle of a later year's growth.
Note the secondary wood and examine the vessels of the spring and autumn
wood.Look for medullary rays.On the outside of the cambium, note the secondary
phloem, containing thickened cells called phloem fibres.Note how the medullary
rays widen out in the phloem.Near the periphery, look for cork cambium,
and note the layers of cork cells produced on the outside of this.
2. Examine a secondarily thickened stem by means of radial longitudinal
and tangential longitudinal sections.
Observe the following tissues:
2.1 The periderm consists of phellem and phellogen.Phellem (cork) cells
have radial rows of cells with suberized walls, formed by division of the
phellogen (cork cambium) one row of
radially flattened cells with thin
walls.In some plants the phellogen may also produce a farther layer, the
phelloderm, towards the inside.
This layer is not apparent in Tilia.The lenticels, part of periderm,
are regions of rounded, loosely packed cells, which allow exchange of gases
through the otherwise
impermeable tissue
2.2 The secondary phloem, in wedges, consists of alternating bands of
fibres and sieve tubes, companion cells and parenchyma.
2.3 The cambial zone
2.4 The secondary xylem, wood, (Greek: xylon, wood)
2.5 Primary rays extend from the cortex to the pith and are very wide
in secondary phloem.
2.6 Secondary rays are in secondary xylem and phloem and do not extend
to the centre or to the cortex.
2.7 Primary xylem surrounds the medulla.The interpolation of secondary
vascular tissues between primary xylem and primary phloem creates considerable
stress in the stem.
3. The medulla and primary xylem are least affected.In the outer layers,
accommodation to the increasing circumference is accomplished by the following
changes:
3.1 The epidermis initially keeps pace with growth by radial cell divisions
but eventually is replaced by a periderm of superficial origin,
usually
just beneath epidermis.
3.2 The cortex increases in circumference by expansion of cells in the
tangential plane, and divisions in the radial plane.Parenchyma is mostly
squashed, but collenchyma retains its form and the new walls from recent
cell divisions are evident.
3.3 Parenchyma cells of the primary rays between phloem wedges expand
and divide similarly.Some of these rays become conspicuous by great increase
in width towards the periphery.9.64 Wood cells, Eucalyptus,
poplarSee diagram 9.57.2: Section of cut wood Be careful! Do not allow students to use
concentrated nitric acid.Use safety glasses and nitrile chemical-resistant
gloves when working with concentrated acids.
Prepare woody elements for microscopic examination by maceration of a
small woody twig.In a fume cupboard, fume hood, cover the pieces with concentrated
nitric acid, add crystals of potassium nitrate, and heat the
beaker in a
fume cupboard.When the reaction has finished, remove all the acid by repeated
rinsing with water.Mount twig tissue in 50% alcohol / water mixture and
tease it apart with mounted needles.Observe vessels with characteristic
thickening on the walls, wide lumens (internal spaces) and perforated end
walls.Observe xylem parenchyma fibres and tracheids, long narrow cells with
lignified walls and narrow lumen.9.65 Leaves of bushy plantsSee diagram 9.65.1: Parts of a leaf | See diagram 9.65.2: Mung bean leaves9.65.3.1: VS of leaf showing water movement1. Examine the leaves of a bushy plant.
Most leaves are flat and thin to catch plenty of sunlight.
The bushy plant leaf has the following 3 parts: 1.1 lamina, 1.2 petiole, 1.3 leaf base.
The leaf base attaches the leaf to the stem.
The petiole turns and holds up the leaf blade, like a handle.The leaf
blade takes in sunlight to make food.Leaf veins are arranged in a
network.A leaf is attached to the stem.In the angle between the leaf
and stem is the axillary bud.A leaflet is part of a leaf.There is no axillary bud
between the leaflet and the stem.2. Choose a simple form of leaf and examine its external appearance in
detail.Note the leaf, showing the swollen leaf base, the petiole and
the lamina.Examine the type of venation, and note how the veins
gradually diminish in size, until the ultimate branches are scarcely visible.
3. Cut a vertical section of a leaf or examine the tissues in a prepared slide.9.66 Leaves of agricultural
plantsSee diagram 9.66: Papaya leaf
Examine leaves of agricultural plants, e.g. banana, breadfruit, chilli,
cassava, cocoa, coconut, pineapple, swamp taro, sweet potato, yam.Draw
each leaf and label the parts and describe the leaf in your own words.For
example, the lamina of the leaf of the Papaya plant has tooth-shaped
lobes and the petiole is long and hollow.9.66.2 Young root, black mustard,
white mustard
Cultivate mustard seed on damp absorbent paper.Use a hand lens or low
power of a microscope to see the cylindrical radicle, the root cap and the
root hairs.Cut off the radicle then cut it longitudinally down the middle
and mount in water.Observe a single mature hair.It is an outgrowth of
an epidermal cell.9.66.3 Structure of dicotyledon
leaf, privet, lilacSee diagram 9.65.7: Privet leaf
1. Examine leaves on a light table.Draw the outline of the leaf and
include the mid rib and veins.
2. Choose a part of a leaf that contains some of the midrib.Fix it
between two pieces of elder pith or carrot so that you can cut the midrib
transversely.Mount the section and stain with an aniline salt.Examine
the structure under the low power, noting the upper epidermis covered with
a layer of cuticle, palisade tissue, spongy tissue,
and lower epidermis
covered with a slightly thinner layer of cuticle.Note also the vascular
bundle forming the midrib, composed chiefly of xylem and phloem.Note the
midrib embedded in a sheath of parenchyma cells and the thin portion of the
leaf.Note the absence of chloroplasts in the upper and lower epidermis,
a large number in the palisade mesophyll cells and a relatively
smaller
number in the spongy mesophyll cells.Note the shape and position of the
chloroplasts.Examine the shape of the cells of the palisade tissue and
note the number of layers here.The cells are separated by small air spaces.
Note the irregular shape of the spongy mesophyll cells and the air spaces
between them.Privet is a mesophyte dicotyledon with dorsi-ventral leaves.
3. Observe the following:
1. The upper epidermis with stomates, cuticle and glandular hairs.
2. The palisade mesophyll consisting of vertically elongated cells with
chloroplasts.
3. The spongy mesophyll consists of loosely packed cells with air spaces
and chloroplasts.
4. The lower epidermis has stomates, cuticle and glandular hairs.
5. Small lateral veins, often cut obliquely during preparation of the
microscope slide, are situated between the palisade and spongy
mesophyll.
6. The midrib is a continuation of vascular tissue of the leaf stalk.
In the midrib vein, the xylem is uppermost and the phloem is on the underside.9.67 Grass leafSee diagram 9.67: Grass leaf
The grass leaf has three parts:
1. leaf blade
2. leaf sheath and 3. ligule,
an outgrowth where the leaf blade and sheath join.The leaf veins are arranged
in parallel.
Cut a vertical section of a grass leaf or study a prepared slide and
observe the following:
1. upper and lower epidermis, chlorenchyma.
2. lack of differentiation into palisade and spongy mesophyll.
3. small lateral veins cut at right angles because of parallel venation.
4. sclerenchyma forming L-shaped girders around lateral veins and midrib,
5. border parenchyma surrounding the veins.9.68 Isobilateral leaf, EucalyptusSee diagram 9.65.8: Eucalyptus, T.S.
When Eucalyptus leaves are isobilateral, twisting of the petiole
allows the leaf to hang vertically and show many xeromorphic
characters,
e.g. thick cuticle and sunken stomates.Observe the mesophyll with palisade
cells, like fence palings, next to both surfaces and the oil glands seen
as large circular cavities.9.69 Stomates in a leafSee diagram 9.69: Stomate, T.S. and V.S.
1. Use a one-sided razor blade to
make an incision on the lower surface of a soft leaf and use forceps to
strip off a small section of
epidermis.Be careful! Cut away from the body!
Mount the strip in water on a microscope slide with the outer surface uppermost.
Most soft leaves have no stomates in the upper epidermis.A stomate is a
small pore surrounded by two kidney-shaped guard cells.The guard cells usually
contain s but epidermal cells do not usually contain chloroplasts.Put salt
solution on the stomate and see if the guard cells become plasmolysed and
so close the pore of the stomate.Plasmolysis is the contraction of the
protoplasm away from the cell wall due to loss of water through osmosis.
The stomate should be open if the weather is bright and sunny. 2. Cut a transverse section of the leaf and look
for a stomate cut in section.Notice the shape of the guard cells and the
pore.Note also the large air space in the mesophyll immediately adjoining
the stomate pore. 3. Pour collodion on the lower surface of a leaf.
Wave the leaf in the air until the collodion dries, then pull it off as
a strip.See and feel the shape of the stomates in the leaf.4. Note the distribution of stomates on leaves of
Tradescantia.Use a glass rod to transfer one drop of water from
a beaker on a microscope slide.Detach a leaf from a hanging plant, stretch
it over the index finger of the left hand, outer surface upwards, holding
it in position with
the thumb and middle finger.Make a scratch along the
surface of the leaf with a dissecting needle.Grip one edge of the scratch
with a pair of pointed forceps and detach a small section of the outer skin,
epidermis, from the upper
surface of the leaf.Place it with the outside
surface facing upwards in the drop of water on the slide and put a cover
slip over it.Using the same technique, prepare a specimen of the outer skin,
epidermis, of the underneath surface of the leaf of the same plant.Examine
both preparations under low power.Note whether stomates are distributed
in similar numbers on the upper and lower surface of the leaf.Examine three
different areas of each specimen.Count how many stomates can be seen in
the microscopic field of vision in each case and calculate the mean values.
Compile a table of results.5. Note the distribution of stomates on leaves of
iris.Use the same technique to prepare a specimen of the outer skin,
epidermis, of both sides of an iris leaf.Examine the specimens under low
power.Note the distribution of stomates on the different sides of the leaf.
Examine three different areas of each specimen as above.Calculate the mean
values.6. Note how the stomates are distributed on either
side of the following: a Tradescantia leaf, an iris leaf.Note whether
the iris leaf has an upper and a lower surface.Examine the stomates of
the iris leaf under greater magnification.Compare their structure with that
of the stomates of the hanging plant, and note whether both structures are
identical.Investigate the distribution of stomates on both sides of the
leaves of other terrestrial plants, e.g. Eucalyptus, an isobilateral
leaf.7. Investigate the distribution of stomates and
its relation to the plant environment.The stomates are responsible for most
of the water diffusion and gas exchange that occur in a plant.So the distribution
of stomates on the leaves is adapted to the environment of the plant.Investigate
the distribution of the stomates on both sides of a water lily leaf and a
leaf of curled pond weed.8. Put a drop of water on a microscope slide.Stretch
a water lily leaf with its outer surface upwards over the index finger
of the left hand, holding it in position with the thumb and
middle finger
of the right hand.Make a scratch along the surface of the leaf with a
dissecting needle.Be careful! Do not press too hard and puncture through
the leaf into the skin.Grip one edge of the scratch with a pair of pointed
forceps and detach a small piece of the epidermis from the upper surface
of the leaf.Put the piece of epidermis in the drop of water on the slide,
with the outside surface facing upwards and put a coverslip over it.Use
the same technique to prepare a specimen of the epidermis, of the underneath
surface of a water lily leaf.Examine both preparations under a microscope.
Compare how the stomates are distributed in the upper and lower sides of
the water
lily leaf.
9. Use the above technique to prepare a piece of the epidermis from
both sides of a leaf of curled pond weed.Examine these preparations under
low power.
10. Stomates control the movement of gases in and out of a leaf, making carbon
dioxide available for photosynthesis, and controlling
the loss of water from
the leaf through transpiration.The density of stomates varies between monocotyledons
and dicotyledons, between plant species, and between the underside and top side
of the leaves.
Experiment
Measure stomate
densitySee diagram 9.69.1: Surface view of the lower epidermis of Kalanchoe
1. Use clear nail varnish, e.g. "Germolene New Skin, Water-based Varnish", to make an impression of the epidermis, Diagram 9.69.1 shows a surface view of the lower epidermis of the dicotyledon Miracle Leaf, (Kalanchoe pinnata).Note the three stomates and their associated guard cells.Each stomate is surrounded by two sausage-shaped guard cells, which
change shape to control the size of the stomate aperture.In the majority
of leaves with an upper and lower surfaces, dorsiventral leaves, like this dicotyledon,
most stomates occur in the lower
epidermis.They are usually evenly distributed
in the leaves of monocotyledons.The stomates of most species open in daylight and close in the dark.Those
plants that use CAM photosynthesis, an adaptation to reduce water loss in
arid conditions, where the stomates close during
the heat of the day to reduce evaporation / transpiration,
and open at night to absorb carbon dioxide for use in photosynthesis.
The guard cells contain chloroplasts, visible in this image, but in most
plant species they are not able to carry out the full process of
photosynthesis.
The wavy blue lines, looking rather like a jigsaw puzzle, are the cell walls
of the epidermal cells.Guard cells develop and differentiate from epidermal
cells.
Selecting your plantsKniphofia uvaria, red hot poker, is one of the best plants for doing
epidermal peels.It is a monocotyledon with its stomates ordered in rows.
They are large and so are they are easy to see when observing the opening
and closing of stomates using solutions of different concentrations.Bergenia crassifolia, elephant-eared saxifrage, leaves also peel very
easily, but the stomates are smaller although clearly visible at
x100 magnification.
It is a dicotyledon so the distribution of stomates is more random.1.
Use clear nail varnish to measure stomate density.However, some leaves are
damaged by the solvent in the nail varnish.Prepare an epidermal impression
by coating the leaf surface with nail varnish.Peel off the dried layer of
nail varnish with Cellotape and stick this onto a slide.With some plants
you can peel off an epidermal strip directly, then mount it in water on a
slide and place under the microscope.Use an eyepiece graticule to count
the number of stomates within different squares to act as replicates, or
working at a higher
magnification and count a number stomates in the area
visible under the microscope.Calculate the area of leaf to give a quantifiable
result e.g. stomates per square mm.
2. Use "Germolene New Skin" to take the impressions. 3. Use a water-based
varnish, from a supermarket.Paint the opaque varnish thinly on to the leaf
to produce a clear film and leave it to dry and be peeled off the next day.
The stomates and cell walls can then be seen..4. Produce impressions on acetate film, by placing
a leaf in propanone and then pressing it onto the acetate.The plant leaves must have an even surface.
5. Rub a board pen over the surface of a leaf.The solvent-based ink permeates the leaf,
showing up the stomates.This method works with only with certain types
of pen.Record the stomate patterns and density in different plants.Note whether
the density varies over a leaf surface and between different leaves of the
same plan or between different plants of the
same species, e.g. Brassica
oleracea: cabbage, brussels sprout, broccoli, cauliflower, Brassicaceae.
Note whether the density of stomates varies between plants from different
habitats, e.g. cactus and other succulent plants, and
between parent and
offspring.9.69.1 Structure of stomates,
Eucalyptus, Hakea, iris, privet, narcissus, water lily
| See diagram 9.65.3.1: VS Leaf
| See diagram 9.69: Surface view and section view of a
stomate
| See diagram 9.69.3 Surface view Guard cells
| See diagram 9.69.1: Hakea stomate
1. Chose a small portion of the leaf, and tear off the epidermis as a
thin layer.Mount the piece of epidermis flat in water and examine it under
high power.Examine a stomate and surrounding cells as a small pore surrounded
by two kidney shape guard cells.The guard cells usually contain chloroplasts
but epidermal cells do not usually contain chloroplasts.While looking at
a stomate irrigate with salt
solution and see the guard cells become plasmolysed closing the pore
of the stomate.
2. Cut a transverse section of the leaf and look for a stoma cut in section.
Notice the shape of the guard cells and of the pore itself.Note also the
large air space in the mesophyll immediately adjoining the stomate pore.9.70 Leaves containing aerenchyma,
water lilySee diagram 9.70: T.S. Water lily leaf
Leaves of water lilies float on the surface of the water so xylem and
mechanical tissues are reduced and aerenchyma is typically present.Stain
a section in acid phloroglucin solution and mount it in glycerine.
Observe the following parts:
1. Upper epidermis with cuticle and stomates.
2. lower epidermis without cuticle.
3. palisade mesophyll interspersed with elongated sclereids.
4. aerenchyma containing large intercellular cavities and interspersed
with stellate sclereids.
5. veins showing small amounts of xylem.9.70.1 Plants in dry environments,
Acacia, Opuntia
| (9.70.1: Acacia, wattle,
Opuntia, prickly pear)
| (9.5.12 Phyllode, Acacia)
|
(9.69.1 Structure of stomates, Hakea)
| (9.6.8 Xeromorphic stem, spinifex)
| (9.82 Leaf of Hakea, xeromorphic leaf )
| (9.73.1 Apogeotropic roots of mangroves)
Plants growing in places of low rainfall, shallow sandy soil that will
not hold water for long have adaptations in their leaves to cut
down the
loss of water or to store water.9.71.0 Dicotyledon root and
monocotyledon root
| See diagram 9.71.1: T.S. Young root with root
hairs
| See diagram 9.71.2: T.S. Older root, high power
(not the same plant)
The xylem vessels carry water and dissolved substances from the root
up towards the shoot.The sieve tubes in the phloem carry water and dissolved
foods towards the root.The xylem is arranged in a star-shaped pattern with
5 or 4 points.Small protoxylem vessels are at the points of the star outside
the larger metaxylem vessels.Between the points of the xylem star are
groups of sieve tubes and companion cells of the phloem.The xylem and
phloem are surrounded by parenchyma tissue.Within the parenchyma are meristematic
cells of the root cambium that later produce secondary thickening of the
root.The xylem, phloem and parenchyma and the layers of parenchyma cells
surrounding the pericycle, together are called the stele or
vascular cylinder.
The outer layer of the vascular cylinder is the pericycle that later forms
fibres.The cortex is the outer cylinder of parenchyma and intercellular
spaces.The innermost layer of the cortex forms the endodermis, a layer
of cells that controls movement of solutions into and out of the stele.Later,
the endodermis has suberized thickening but cells opposite the protoxylem
groups, remain thin-walled and are called passage cells.The outermost layer
of the root is the piliferous layer that produces root hairs.Lateral roots
originate in the pericycle.Monocotyledon roots have much pith and many scattered
xylem and phloem bundles.Experiment
Cut a transverse section of a broad bean root.Use the thumb and forefinger
to hold the root between two pieces of pith or carrot tissue.Dip the material
in water to moisten it.Never cut dry material.To cut microscope sections
use a one-sided razor blade dipped in water.Hold the material vertically
and draw the razor blade quickly across it away from the body.Cut the thinnest
possible sections as wedge shapes.Wash the sections into a small dish
of water.Use a camel hair paint brush to select the thinnest section, mount
it in water on a microscope slide and apply a coverslip.Irrigate the section
with aniline sulfate to colour the xylem elements yellow.
Observe the following:
1. The piliferous layer, with its root hairs.
2. The cortex, composed of the cortex proper and the endodermis.
3. The stele, composed of xylem (protoxylem and metaxylem) phloem and
pericycle.These tissues are all embedded in parenchyma.
4. Note the relative positions of the various tissues.Examine the tissues
under high power and note the cellular structures.9.72 Legume roots, broad bean,
clover, Rhizobium
| See diagram 9.72: Legume root and root nodules
| See diagram 9.209: T.S. Root nodule
| See diagram 9.72.1: Winged bean, pigeon pea, mung bean
Look for root nodules on legumes, clover.Prepare a transverse section
of such a root to pass through a nodule.Note the red colour usually shown
by the central part of the nodule and study the large infected cells present
in this region.9.73 Gravity affects the growth
of stems and roots
1. Sprout seeds and select one that is straight.Pierce the seed with
a long pin or needle and stick this into a cork.Put damp cotton or absorbent
paper in a bottle.Put the cork and seedling in the bottle.Put the bottle
in dark cupboard and look at it every four hours.
2. Put seeds that grow rapidly, oats, radish, or mustard seeds on moist
absorbent paper between two glass plates secured with
rubber bands.After
germination, turn the apparatus vertically through 90o and leave
to remain undisturbed.Repeat the turning at intervals and observe the effect
on the roots.
3. A technique used in some laboratories is to put developing plants
in a shaking machine.Without a sense of gravity the plants the tissue,
e.g. orchid tissue plantlets, does not differentiate into roots and shoots.
This process allows undifferentiated tissue to grow bulkier and prepare
for transplanting.9.73.1 Apogeotropic roots
of mangrovesSee diagram 9.73: Mangrove roots
Observe mangroves in tidal swamps.They have apogeotropic roots containing
aerenchyma.The roots grow upwards.Aerenchyma gas spaces provide an internal
passage for oxygen gas in plants growing in flooded and anaerobic habitats.9.74 Excretion of acids by rootsSee diagram 9.74: Excretion of acid by roots
Plants excrete acids through the roots.These acids dissolve the otherwise
insoluble chalky constituents of the soil.Put a marble plate or bathroom
tile with the polished side upwards in a sloping position in a flowerpot.
Fill the flowerpot with soil.Plant a bean seedling with roots about 2 cm
long in such a position that the roots are forced to grow along the polished
surface.After three weeks, remove the marble plate and rinse it with water.
The polished surface of the marble plate has become etched
where it was
in contact with the roots.To make the etched lines clear, apply black shoe
polish with a pad of cotton wool.The acids excreted by the roots of the
bean plant have dissolved the marble, calcium carbonate, at the points of
contact.9.75 Root hairs of germinating
bean plantSee diagram 9.75: Root hairs of germinating bean
seed
Put bean seeds or mustard seed on damp absorbent paper.Cover the seeds
to keep the paper damp.After germination, use a magnifying glass to examine
the cylindrical radicle, the root cap and the tiny root hairs growing from
the side of
the root just behind the root tip.They are very thin walled
outgrowths of the epidermal cells.Most plants take water and plant nutrients
into their roots through the root hairs.Root hairs may be damaged by careless
transplanting, salty soil and lack of oxygen in waterlogged soil.9.76 Root structure of mung
beanSee diagram 9.72.1: Mung bean plant
Wash the soil from the roots of a small bushy plant, e.g. mung bean,
and a grass, e.g. para grass.Bushy plants have a main root, the tap root
or primary root, and smaller lateral roots or secondary roots.These roots
can grow very deep.Grasses and palms have no main root, only many fibrous
roots.These are thin roots and do not grow deep.9.77 Roots absorb water, TradescantiaSee diagram 9.77: Roots absorb water Put a rooted and an unrooted shoot of Tradescantia each
in a test-tube.Select the shoots so that they have the same leaf area.
Fill two test-tubes with tap water to 2 cm below the rim.Pour a thin layer
of paraffin oil on the water in each test-tube.Mark the height of the
surface of the liquid on the test-tube with a felt pen.Put the test-tubes
in a test-tube rack.Record the level of water in the two test-tubes each
day.The surface of the liquid drops slightly in the test-tube containing
the unrooted Tradescantia shoot, but drops more in the other test-tube
containing the well-rooted shoot.9.78 Celery stalk, bean stem
| See diagram 9.78: T.S. Celery stalk
| See diagram 9.3.60: T.S. Bean stem
Celery stalks are enlarged petioles.A petiole is not a stem.The stem
of the celery plant is reduced to a disc.Experiment
Wear safety glasses and nitrile chemical-resistant
gloves.Use a one-sided razor blade to
cut thin transverse sections and longitudinal sections from a celery stalk.
Be careful! Cut away from the body.Mount in water and apply a coverslip.
Stain sections with acid phloroglucin.Observe the following tissues:
1. Epidermis is a single layer of cells with a thick cuticle covering the outermost surface.
2. Collenchyma has cellulose thickenings in the corners of the cells.
3. Parenchyma has large cells with thin cell walls.
4. Vascular tissue is arranged as discrete collateral bundles.Each vascular bundle
has phloem to the outside, xylem to the inside and cambium between.
5. Sclerenchyma caps the vascular bundle.9.79 Dicotyledon stem, sunflowerSee diagram 9.79: T.S.Young sunflower stem |
See diagram 9.78.1: Sunflower Stem LS
1. Mount the section and irrigate with an aniline salt, to stain the
woody tissues.Examine the whole section under low power.Observe the epidermis,
cortex and central cylinder.Look for any layers of the cortex thickened
to give additional strength.Observe the vascular bundles composed of xylem,
phloem and cambium.Observe the pith and note if the stem is solid or hollow.
2. Observe the stem under high power and note the cellular structure
in detail.Observe the epidermal tissues, cortical tissues and a complete
vascular bundle.
3. Examine a similar stem by means of radial longitudinal sections,
R.L.S.Note the appearance of the collenchyma, the annular protoxylem vessels,
and the pitted metaxylem vessels.9.80 Monocotyledon stem, maize,
(corn), Zea maysSee diagram 9.80: T.S. stem of maize (corn)
Use low power to observe the small size, large number, and irregular
arrangement of the vascular bundles.The outer scattered vascular bundles
are surrounded by fibres to strengthen the stem.Note the complete absence
of cambium.9.80.1 Monocotyledon stems,
Cocos nucifera coconut, Dracaena
| See diagram 9.78.4: Dracaena
| See diagram 53.4: Cocos nucifera, coconut9.82 Leaf of Hakea, xeromorphic
leafSee diagram 9.69.1: Hakea stomate
Many Hakea species have needle-shaped leaves in which there is
a reduction in leaf surface / volume ratio.
Observe the following:
1. epidermis with thick cuticle and sunken stomates.
2. palisade mesophyll cells in a double layer interspersed with sclereids,
3. central storage mesophyll with scattered vascular bundles.9.97 Roots from plant partsSee diagram 9.85: Potato
Obtain a box of sand and put it out of direct sunlight.Wet the sand
thoroughly and keep it moist.
Plant any of the following in the sand:
1. various bulbs.
2. cuttings of begonia and geranium stems.
3. a section of sugar cane stem with a joint buried in the sand.
4. a section of bamboo stem with a, joint buried in the sand.
5. carrot, radish and beet tops, each with a small piece of root attached
6. an onion.
7. an iris stem.
8. pieces of potato containing "eyes".
9. a branch of willow.Table 9.0.1 Plant tissue types